The present invention relates to a printing system and, more particularly, to a counter pressure cylinder having an elastic surface and being adapted to press a paper or web on a surface of a printing cylinder of a printing unit, for example, for photogravour printing, heliogravour and intaglio printing equipped with an electrostatic printing system.
Printing assist systems of the aforementioned type have been proposed in various forms, with the printing assist system serving for a buildup of an electrostatic field in a pressure nip between the pressure or form roller associated with a printing cylinder in order to effect an improved ink transfer to a web, such as, for example, a paper web, traveling in a pressure nip between the counter pressure cylinder and the printing cylinder of the printing unit.
A disadvantage of all printing assist systems resides in the fact that the manufacturing thereof is expensive and already existing printing units for the most part can no longer be retrofitted with such assist systems.
The aim of the present invention essentially resides in providing an electrostatic printing assist system which is simple in construction and which is also capable of being retrofitted or added t existing printing units.
In accordance with advantageous features of the present invention, the counter pressure cylinder is provided, on an insulating jacket thereof, with a coating that is optionally homogeneous but conductive and preferably metallic, with a further coating of a limited conductivity being arranged thereover. A primary winding is disposed concentrically to the electrically grounded axle or shaft of the counter pressure cylinder adjacent the counter pressure cylinder in a fixed fashion with respect to the printing unit. A secondary winding is arranged concentrically to the electrically grounded axle or shaft of the counter pressure cylinder on one of its end faces in a fixed fashion with respect to the counter pressure cylinder, with one of its connections being connected to the axle or shaft and the other connection being connected to the coating by way of a rectifier circuit.
By virtue of the features of the present invention, in principle, a bipartite transformer is utilized, with the primary side of the transformer being fixed to the printing unit and the secondary side thereof being affixed to the counter pressure cylinder for rotation therewith. A voltage induced in the secondary winding is rectified and smoothed by means of a smoothing circuit and then transmitted to the metallic coating of the counter pressure cylinder.
Advantageously, electric charges are produced which are not linked to the conductor and no electrodes for charging the surface of the counter pressure cylinder are required so that use in explosion-protected localities appears particularly suitable.
Additionally, with the constructional features of the present invention, conventional printing units can be retrofitted in a simple manner by utilizing only a counter pressure cylinder constructed in accordance with the present invention and the secondary winding is mounted on one of its end faces with the primary winding being arranged on the axle and/or shaft of the counter pressure cylinder in dependence upon whether the printing cylinder is driven about the axle or revolves rigidly joined thereto as a shaft.
In accordance with the present invention, by virtue of the provision of two additional windings, a voltage effective on a secondary side can be back induced in a simple manner so that, by way of this measurement transducer, control of the induced voltage in the secondary winding is made possible.
In accordance with additional perfecting features of the present invention, the primary and secondary windings, as well as the second primary and second secondary windings may include a number of component windings each of which is wound up on, for example, bar magnets, with the bar magnets lying in mutual opposition through the nip. All of the magnets preferably extend in parallel in axis of the printing cylinder and are located in mutual alignment.
Moreover, according to the present invention, the bar magnets associated with the primary winding and the second primary winding are arranged at differing radial distances from an axis of the counter pressure cylinder and this may also hold true for the secondary winding and the second secondary winding. By virtue of this arrangement, magnetic induction is generated by a relative velocity between the primary and secondary windings.
Additionally, in accordance with the present invention, it is also possible for the coating to be connected by way of a voltage multiplier circuit.
Advantageously, the smoothing circuit of the present invention for the pulsating direct current is arranged between the rectifier circuit and the metallic coating.
The primary winding is associated with a second primary winding and the secondary winding is associated with the second secondary rotating with this winding and connecting in parallel thereto, with the secondary winding lying in opposition to the second primary winding.
Advantageously, the primary winding can be connected to a source of AC voltage with a frequency of 10.+-.5 kHz, with the source being variable by way of a control unit.
According to the invention, the second primary winding may be connected to the input of the control unit and a plurality of first bar magnets may be provided in place in of the primary winding.
The primary winding may, according to the present invention, include a number of component windings corresponding to the number of bar magnets, with the component windings being wound upon the magnet. Likewise, the secondary winding may include a number of component windings corresponding to the number of first bar magnets, with these components also being wound upon the second bar magnets arranged in alignment with the first bar magnets.
It is also possible for the second primary winding to include a number of component windings corresponding to the number of third bar magnets, with each of these component windings being wound up on one of the third bar magnets.
The second secondary winding may include a number of component windings corresponding to the number of third bar magnets, with the component windings being wound up on fourth bar magnets arranged in alignment with the third bar magnets, and the secondary winding may be connected to the coating by way of the voltage multiplier.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purpose of illustration only, several embodiments in accordance with the present invention.